:%s/foo/bar/gci Change each 'foo' (case insensitive due to the i flag) to 'bar'; ask for confirmation. :%s/foo\c/bar/gc is the same because \c makes the search case insensitive. This may be wanted after using :set noignorecase to make searches case sensitive (the default).

:%s/foo/bar/gcI Change each 'foo' (case sensitive due to the I flag) to 'bar'; ask for confirmation. :%s/foo\C/bar/gc is the same because \C makes the search case sensitive. This may be wanted after using :set ignorecase to make searches case insensitive.